1. Field of the Invention
The present invention generally relates to a power semiconductor switch. Specifically, the invention is a power switch in which a double-gated, normally-off, low-voltage lateral junction field effect transistor (LJFET) is employed to control a normally-on, high-voltage vertical junction field effect transistor (VJFET).
2. Background
An ideal power semiconductor switch possesses important features, capabilities, and characteristics. A normally-off feature is significantly important to power systems since it avoids the dangers and design complications of normally-on designs, especially at high power levels. A voltage control capability is a widely desired advantage thereby spurring the replacement of silicon (Si) power BJTs and Darlingtons current-controlled devices by voltage-controlled FETs (MOSFETs) and IGBTs and more recently the replacement of Si HV-IGBTs by Si GTOs. A negative temperature coefficient characteristic ensures higher reliability by forcing device current to decrease when a local hot spot develops thereby preventing thermal runaway. A low forward voltage drop characteristic ensures low conduction loss and heat generation. A high speed capability reduces both size and weight of reactive components in a power system. Unipolar operation ensures negligible storage charge resulting in low switching losses and reduced switching spikes therefrom higher system reliability.
It is well known that Si IGBTs, bipolar switch types, dominate the higher power end of the commercial market because Si MOSFETs, unipolar switch types, lack conductivity modulation and possess a higher forward voltage drop than Si IGBTs. However, there is no such problem with silicon carbide (SiC) unipolar MOSFETs and JFETs because higher electric field strength and higher doping concentrations make it possible to reduce the specific ON resistance by a few hundred times.
Power semiconductor switches are generally required to handle high power inevitably leading to ohmic losses and heating. Switch heating influences the ultimate limit of power handling capability. In order to operate at high temperatures, the semiconductor requires a large band gap so that the intrinsic carrier concentration is not high enough to cause switch malfunction. Wide band gap semiconductors, one example being SiC, are attractive for this reason. SiC is the only wide band gap semiconductor (3.2 eV band gap for 4H-SiC) for which large area substrate is commercially available. Also, it is an indirect band gap semiconductor with a high thermal conductivity, typically ten times that of GaAs and three times that of Si.
SiC power devices have been intensively investigated. Most efforts have been focused on power switches based on MOSFET technology. MOSFET-based power switches have low reliability due to gate oxide failure, especially at high temperatures. A variety of designs have been proposed to address this issue as described in Status and prospects for SiC power MOSFETs, IEEE Transaction on Electron Devices, Vol. 49, No.4, April 2002.
Gate oxide/insulator reliability, especially at high temperatures, is a major problem for devices such as ACCUFETs, IGBTs, MCTs and MTOs that require MOSFETs or MISFETs for gating. Even when SiC power switches are not used in a high temperature environment their junction temperatures are high, especially where limited space is available.
Gate oxide/insulator reliability at high field and temperature conditions is a much more challenging problem in comparison to the low inversion layer mobility problem hindering the application of SiC MOSFET-based power switches because silicon dioxide (SiO2) on SiC is reasonably assumed to have a quality similar to SiO2 on Si, which does not have carbon atoms to compromise the quality of SiO2. Si/SiO2 technology has a 2 MV/cm maximum field limit in SiO2 so as to achieve a 10-year reliability. Whereas, a SiC/SiO2 structure requires at least a 4 MV/cm field in SiO2 in order to exploit the advantage of high electric field strength offered by the SiC.
SiO2/SiC structures have an intrinsically lower reliability than SiO2/Si structures due to a much larger charge injection into SiO2 from SiC resulting from a smaller injection barrier, 2.70 eV for 4H-SiC/SiO2 barrier versus 3.15 eV for Si/SiO2 barrier. The exponential dependence of the hot carrier density distribution on energy makes 4H-SiC/SiO2 systems more susceptible to high temperature reliability failure than Si/SiO2 systems. Therefore, the need exists for power switches free of gate oxide or gate insulator for high temperature and high power applications. Thyristors, GTO thyristors, and BJTs are free of gate oxide or insulators, but are latch-on devices or current controlled switches and not as desirable for many power control applications.
While novel MOSFET designs may substantially reduce the electric field across the gate oxide in the channel region, there remains a high electric field across the gate oxide in source-to-gate and drain-to-gate overlap regions. Source and drain regions are generally subjected to ion implantation techniques and high temperature annealing. Oxide quality in such regions is substantially degraded causing further reliability concerns with the MOSFET-based SiC power switches within high temperature operations.
Junction field effect transistors (JFETs) have been proposed to solve the gate oxide reliability problem at high electric field and temperature conditions as described by Zhao in U.S. Pat. No. 6,107,649 issued Aug. 22, 2000 and U.S. Pat. No. 6,423,986 issued on Jul. 23, 2002. Power JFETs are typically normally-on devices and as such not as desirable for high power system applications. See R. N. Gupta et al., A 600 V SiC Trench JFET, Materials Science Forum, Vols. 389-393, pp. 1219-1222, 2002; H. Onose et al., 2 kV 4H-SiC Junction FETs, Materials Science Forum, Vols. 389-393, pp. 1227-1230, 2002; P. Friedrichs, et al., Application-Oriented Unipolar Switching SiC Devices, Materials Science Forum, Vols. 389-393, pp. 1185-1190, 2002. However, JFET designs are either normally-on which is not desirable for power switching, see Friedrichs et al., Gupta et al., and Onose et al., or require epitaxial regrowth which not only increases cost but also fabrication complexity thereby limiting device yield, see Friedrichs et al. and K. Asano et al., 5 kV 4H-SiC SEJFET with Low RonS of 69 mΩcm2, Proc. 14th Int. Symp., Power Semiconductor Devices and IC's, Piscataway, N.J., IEEE, 2002. Epitaxial regrowth is an undesirable approach not only because it is costly but also due to its effect on decreasing power device yield.
What is required is a junction field effect transistor (JFET) that is normally-off, offers improved performance over the related art, and eliminates epitaxial regrowth during device fabrication.